The present invention relates to a process for preparing polytetrafluoroethylene (hereinafter referred to as "PTFE") granular powder.
The molding powder of PTFE is a PTFE powder prepared by finely dividing coarse particles obtained by suspension polymerization, and it has been used for molding by means of compression molding or ram extrusion molding. The particle size of the finely devided PTFE particles (primary particle size) is 5 .mu.m at the smallest and up to about 1,000 .mu.m, usually has an average particle size of not more than 100 to 200 .mu.m.
The PTFE powder is used in the form of powder as it is, because PTFE is different from thermoplastic resins or melt-processable resins, and cannot be thermally processed. Accordingly, the particular powder characteristics are required for the PTFE molding powder. One of the characteristics is a good powder flowability, and others are a high bulk density, less destructible and not too brittle.
Generally, the PTFE primary powder is used for molding by agitating the powder in a liquid medium to agglomerate and then granulating, because the primary powder does not have the above-mentioned powder properties. The method for granulating is roughly classified into a dry mixing method or a wet mixing method. The former is a method using a water-insoluble organic liquid alone and the latter is a method using water a two-phase liquid medium of water and a water-insoluble organic liquid. The wet mixing method is superior to the dry mixing method because an automation of granulation process can be relatively easy, though a separation step and a drying step after the aqueous treatment are required.
The present invention relates to the latter method, i.e. the wet mixing method. Hitherto, suitable organic liquids used in the wet mixing method are organic liquids having a surface tension of not more than 35 dynes/cm at 25.degree. C. and a boiling point of about 30.degree. to 150.degree. C. (Japanese Examined Patent Publication No. 22619/1969, No. 40099/1979, Japanese Unexamined Patent Publication No. 18730/1982, etc.). Examples of the organic liquids are chlorofluorohydrocarbons as well as aliphatic hydrocarbons and aromatic hydrocarbons. Examples of the chlorofluorohydrocarbons used in the conventional method are perhalohydrocarbons such as trichlorotrifluoroethane, monofluorotrichloromethane, difluorotetrachloroethane, Cl(CF.sub.2 CFCl).sub.n Cl, and trichloropentafluoropropane. In the actual plants, the perhalohydrocarbons are mainly used.
The perhalohydrocarbons are preferable as the organic liquid used in the wet mixing method in view of incombustibility and recovery of solvent. However, since the perhalohydrocarbons have a relatively high boiling point (80.degree. to 130.degree. C.), the recovering temperature becomes higher, and the granular powder is exposed to a high temperature, which makes the resultant particles hard. The hard granular powders provide a molded article having a rough surface and a low tensile strength, elongation, gas permeability and electric insulation. Further, there is a disadvantage of high energy cost for recovering. In addition, it is said that the perhalohydrocarbon is one factor of destroyment of the ozone layer, and therefore use of the perhalohydrocarbon is not desirable.
As a result of the present inventor's intensive study in order to find an organic liquid being capable of solving the above-mentioned problems instead of the perhalohydrocarbons, the particular halogenated hydrocarbons having 2 to 3 carbon atoms, at least on fluorine atom and at least one hydrogen atom are commercially advantageous, and have excellent in powder properties and low influences to the ozone layer, as well as give a molded article improved in various properties, then the present invention has been completed.